Film resistor element



Feb.

D. W. MOORE El'AL FILM RESISTOR ELEMENT Original Filed Nov. 4, 1954 L I A, AA 2 r f I l l l 1 1 1 I, 1 1 1 I I, 1 I, 1 1 1 1 1 1 e/Mou DAVID W. MOORE ORA F. KUHLMAN WWW United tat-es Patent FILM RESISTOR ELEMENT David w. Moore, Pacific Palisades, and Ora F. Kuhlman,

Manhattan Beach, Calif., assignors to Servomechanisms In"c., Hawthorne, Calif., a corporation of New. York 5 Claims. c1. ass-30s This invention relates to film resistor elements for use in adjustable resistor devices and, more particularly, to precision film resistor elements having a high degree of stability over a wide range of environmental conditions and over a long life cycle.

This application is a division of David W. Moore and Ora F. Kuhlman application Serial No. 466,806, filed November 4, 1954, now Patent No. 2,827,536, originally filed for- Film Resistor Element and Method of Fabricating the Same, and now limited to the Method of Fabricating Film Resistor Elements.

Resistor elements comprising an extremely thin film of stable non-oxidizing metal thermally evaporated in a vacuum upon an insulation form, such as a glass plate, have been found useful in adjustable resistor applications requiring a precisely predetermined resistance-displacement characteristic stable over a wide range of ambient conditions and over a long life cycle. Because of the extreme thinness of such metal films, sometimes of only .a few molecules thickness, a difficulty has been encountered in utilizing a contact element or brush bearing directly on the film since the wear of a film by such a contact element, while microscopic, may nevertheless be an appreciable fraction of the film thickness, thus substantially changing the resistance characteristic of the unit. To avoid such deterioration, another type of re-, sistor element has heretofore been proposed in which such a resistance film is superposed upon a commutator structure and contact is made with portions of the commutator elements extending beyond the resistance film. Resistor elements of such construction are described in and claimed in the patent of D. W. Moore, Jr., Reissue No. 23,219, andin the co-pending application of I). W. Moore, Jr., Serial No. 151,430, filed March 23, 1950, now United States Letters Patent 2,720,512.

While metallic film resistor elements of the types described are reasonably satisfactory for certain applications, they are subject to a number of limitations. For example, resistor elements of the first type are usually 7 formed von a base member of plastic or glass or other dielectric material having a comparatively smooth surface.

In such resistor elements, the thickness of the film has been a compromise between that required to obtain the desired resistance per square and that adequate to withstand brush wear for a reasonable life cycle without an undue percentage resistance variation. Specifically, the required resistance per square is frequently of such a value that it can be obtained by a film of practical dimensions only by the use of an extremely thin film, while such a thin film results both in' a normal brush wear giving unacceptable resistance variations over a moderate life cycle and an unacceptable resistance stability characteristic. On the other hand, the elimination of this wear by the use of separate commutator elements, as in the second type of resistor element described, reduces the resolution obtainable and increases the cost and complexity of the unit.

' It is an object of the present invention, therefore, to

ice

7 2 provide a new and improved film resistor element for use in an adjustable resistor device which obviates one or more of the above-mentioned limitations of such resistors heretofore proposed.

It is another object of the invention to provide a new and improved precision resistor element for use in an adjustable resistor device, which maintains a high degree of precision and a stable over-all resistance value over a wide range of environmental conditions and over a long life cycle.

It is another object of the invention to provide a new and improved film resistor element for use in an adjustable resistor device which provides a maximum resistance per square for a film of any given thickness.

It is another object of the invention to provide a new and improved film resistor element for use in an ad justable resistor device, in which abrasion by a contact brush is negligible over a long life cycle.

In accordance with the invention, in an adjustable resistor device, there is provided a resistor element comprising an extended-surface base member of dielectric ma:

impart thereto a granular surface having a predetermined mean grain size, masking such ground surface to expose a desired portion thereof, depositing a thin high-resistance metallic film directly on such exposed surface portion, applying a protective coating of dielectric material to the deposited film, and removing excess coating to expose a multiplicity of minute areas of the film.

By the term high-resistance film as used herein and in the appended claims is meant a film in which the high resistance is determined primarily by the thinness of the film and secondarily by the specific resistance of the ma: terial of which the film is composed. The term grind ing as used herein and in the appended claims is used to include conventional lapping and abrading processes.

For a better understanding of the present invention, together with other and further objects thereof, reference is bad to the following description taken in connection with the accompanying drawing, while its scope will be pointed out in the appended claims.

Referring now to the drawing: 7

Fig.1 is a perspective view of an adjustable resistor device including a resistor element embodying the pres ent invention;

, Fig. 2 is a very much enlarged fragmentary cross sec- Referring now to Fig. 1 of the drawing, there is represented an adjustable resistor device incorporating a re- The resistor sistor element embodying the invention. element comprises a base member 10 of dielectric ma terial which may be in the form of a circular disc of unglazed, homogeneous, non-porous ceramic material, such as steatite. The disc 10 has an extended substantiallyplane surface 10a which is granular 'in character. For

example, this surface 10a may be ground with a gritmaterial such as aluminum oxide or silicon carbide having a grain size within the range of l to 500 microns, the resistance per square varying over a wide range directly with variations in grain size. Formed on the disc 10 are a pair of conductive terminals 11, 12 which maybe this'purpose. protective coating, and the surface 19a is shown in very form thickness of a lower order of magnitude than the mean grain size of the surface lfia, specifically, within a range of a few molecules to a few microns. The resistance film 3.3 is formed of a stable, non-corroding metal itich as a noble metal, a nickel-chromium alloy, or the Superposed upon the film l3 and the exposed surface 10a is a protective coating of dielectric material, which may be deposited as described hereinafter, leaving exposed a multiplicity of minute areas of the film, The dielectric coating material may be a self-hardening drying oil, silicone, varnish, etc., a5 sodium silicate solution, by weight, having been found particularly suitable for The relationship of the'resistance film, the

much enlarged cross-sectional detail in Fig. 2. The 'surface is granular or jagged in character, very irregular in configuration, as shown in Fig. 2 by the dotted-line peaks a, b, c, i, falling in a common plane. The thin metallic film 13 conforms closely to this irregular surface configuration. The thicker protective dielectric coating 14 is somewhat smoother and follows generally the configuration shown by the dotted line 14.1 If the excess of this coating material is then uniformly removed, as de-. scribed hereinafter, .it is levelled off to a plane, such as that representedby the trace, 15, 15, including the in- .dividual peaks of the granular surface .l tla, so that a minute area of the film 13 is just exposed through the dielectric coating 14 at each or" the peaks.

Referring again to Fig. l, the complete adjustable resistor device'includes a brush arm 16 extending from 'a hub 17 pivoted on a shaft 13 to which the disc it} is secured in any conventional manner. Extending from the hub 17 are a pair of strip or Wire biasing springs 19 and it) which engage a graphite brush element 21 and which respectively bias it verticallydownward into engagement with the film strip 13 and against the face of the brush arm 16 to restrict movement of the brush 21 substantially to the vertical plane of the brush arm 16. The general arrangement of the brush 21 and its brush arm 16 forms no part of the present invention, but is described and claimed in the co-pending application of Ora F. Kuhlman, Serial No. 466,805 filed November 4, 1954; forMetallic Film Adjustable Resistor and fiontact Brush Therefor.

In an adjustable resistor device of the type described, it will be noted that the brush element 21, in general; contacts the microscopic exposed portions or peaks of the metallic film 13 but contacts the dielectric coating material 14 over a preponderant area of the resistor element. With this arrangement, operation of the device over a large number of cycles results in a wearing primarily of the graphite brush 21 by the'dielectric coating material, rather than a wearing of the resistance film by the brushv element, as in prior film resistors of this type.

The wearing of the graphite brush 21 is not too important,

since these brushes are inexpensive and may be readily replaced.

The resistor element described may be fabricated by any of a number of processes, but ere follows a description of a preferred process. Initially, the surface lba'of the ceramic base 10 is ground to impart thereto a granular surface having a predetermined grain .size. Specifically, one surface satisfactory for the purposes of this invention has been formed by an initial coarse grind with a grit of silicon carbide of micron grain size followed by; a finishing grind with a grit of aluminum oxide (A1 0 sired limits of the device.

of 225 micron grain size, imparting to the surface 10a a granular configuration having a grain size of the order of 20 microns. The terminals 11 and 12 may be formed by applying a conductive paint and firing, as described in aforesaid co-pending application, Serial No. 151,430.

The surface lt a between the terminals 11, 12 is then sealed or glazed by applying by conventional screening technique a glazing mixture. Any suitable vitreous or ceramic glaze may be used, but a mixture of powdered lead borate and n-butyl phthalate in proportions to form a paste thick enough to screen has been found satisfactory, The base 1% is then fired at a suitable temperature which for the mixture specified is approximately 1()50 F. The member is then thoroughly cleaned with any conventional neutral detergent to ensure that it is substantially chemically clean, The surface 10a is then masked to expose a desired portion thereof corresponding to the configuration of the resistance film and there is deposited on the exposed portion athin high-resistance metallic film. This deposition may beby thermal evaporation in a high vacuum chamber, also as described in aforesaid co-pending application, Serial No. 151,430.

After the formation of the resistance element as de scribed, the protective coating of dielectric material 14 is then deposited on the film. This protective and sealing coating may be applied by apparatus such as illustrated in Fig. 3. The base'member it is mounted on and secured to a shaft 22 which is driven by a motor 23 energized from suitable supply circuit terminals through a speed-regulating resistor 25. The shaft 22 and the base member It are rotatedat a moderate speed, for example, of the order of 8C r.p.m. The surface 19a and the resistance'film T3 are then sprayed during rotation with aliquid self-hardening coating of dielectric material, for example, a 5% solution by weight of sodium silicate. This spraying may be etfected'by a nozzle 26 having a control valve 27. 7 approximately one revolution of the base member 10 and thereafter shut off. Simultaneously, a graphite wiper brush '28 displaced from the nozzle 26 by approximately 180 is biased against the surface 16a and the resistance film 13 by means of suitable springs 29, 29

exerting a pressure against the face of the base member 1% of approximately 50 grams The base inember 10 is then rotated several revolutions after the nozzle 26 is ting-off of the spray nozzle 26 with the wiper brush 28 'in place and approximately 36 additional revolutions with the wiper brush removed. After an interval of time, which may be of the order of 15 minutes, one or more additional coatings may be applied in the same manner.

After the application of one or more sealing and pro-.

tective coatings to the resistance element, as described, it is placed in a dust-proof container for a period of about 3 hours or more at room'temperature to permit it to bethoroughly dried.

The coated resistance element fabricated as described is then preferably aged by applying to the terminals 11, 12 an over-voltage for a period of several days at .an elevated temperature to stabilize its' resistance value. Specifically, one aging process whi h has been found satisfactory comprises the application of a voltage of the order of of the rated voltage for a period of .5 to 20 days at a temperature of the order of 350 F. The application of thisaging voltage is continueduntil its rate of change of resistance with time is within thede- While it will be apparent that a resistor element of the type described maybe formed having a-wide range The spray is continued for" bodying, and constructed in accordance with, the present invention:

Surface a Resistance film 13 mean length; 0.062 inch width. Resistance 50 ohms square; 5,000 ohms, total. Coating 14 Sodium silicate,

5% solution by weight, 40"v Baum.

It will be seen that a resistor element of the type described, due to the greatly increased length of resistance path occasioned by the granular character of the surface 10a, has a much higher resistance per square than prior metallic film resistors. Similarly, electrical contact may be made 'by a brush bearing directly on the resistor film. After the element is abraded during the break-in process, as shown in Fig. 2, further movement of the brush 21 over the resistance film results primarily in a wearing of the brush element by the hard ceramic base material and an inappreciable wear of the resistance film 13, providing a useful life upward of a million cycles. At the same time, substantially the entire surface area of the resistance film 13 is protected by the coating 14 from environmental atmospheric conditions and from oxidizing or corrosive infiuences with no appreciable effect upon the resistance characteristic of the element, thereby providing greatly improved resistance stability.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In an adjustable resistor device, a resistor element comprising: an extended-surface base member of dielectric material, said surface being granular in character; and a high-resistance metallic film formed directly on and bonded to said surface, said film being of a uniform thickness of a lower order of magnitude than the mean grain size of said surface.

2. In an adjustable resistor device, a resistor element comprising: a circular disc base member of ceramic material having a substantially plane surface granular in character; and a high-resistance metallic film formed directly on and bonded to said surface, said film being of a uniform thickness of a lower order of magnitude than the mean grain size of said surface.

3. In an adjustable resistor device, a resistor element comprising: a circular disc base member of steatite having a substantially plane surface granular in character; and a high-resistance metallic film formed directly on and bonded to said surface, said film being of a uniform thickness of a lower order of magnitude than the mean grain size of said surface.

4. In an adjustable resistor device, a resistor element comprising: a circular disc base member of ceramic material having a substantially plane surface granular in character and a high-resistance metallic film formed direct- 1y on and bonded to said surface, said film being of a uniform thickness within the range of a few molecules to a few microns.

5. In an adjustable resistor device, a resistor element comprising: an extended-surface base member of dielectric material, said surface being granular in character; a highresistance metallic film formed directly on and bonded to said surface, said film being of a uniform thickness of a lower order of magnitude than the mean grain size of said surface; and a protective coating of dielectric material superposed on said metallic film and exposing a multiplicity of minute areas of said film.

References Cited in the file of this patent UNITED STATES PATENTS 2,075,377 Varian Mar. 30, 1937 2,144,673 Birdseye et al I an. 24, 1939 2,688,679 Schleuning Sept. 7, 1954 2,757,104 Howes July 31, 1956 

